The detection or sensing of hydrocarbon fuels, such as diesel oil, gasoline and Jet-A fuel leaking from storage tanks has received considerable attention. These hydrocarbon fuels are stored in substantial volume in above ground and below ground storage tanks and present a significant hazard to safety and health if they leak into the surrounding environment.
Two approaches have generally been taken to the problem of hydrocarbon fuel leakage. First, storage tanks may be constructed with double bottoms so that leakage from the inner tank is caught and contained by the outer bottom wall. This approach is very expensive and is often impractical in retrofitting situations. The second approach is to provide detection or sensing apparatus proximate the storage tanks which are capable of sensing leaking fuel from the tanks. Upon detection of leaking fuel, the source of the leak can be found and repaired. These approaches also may be used together.
The detection of leaking hydrocarbon fuels, however, is not without considerable problems. Tanks themselves often are very large and situated in even larger tank farms, making it necessary for a multiplicity of detectors to be used and a premium to be placed on locating the source of the leak. As a plurality of discrete detection apparatus are employed, the detection costs rise rapidly. If multiple detectors are not used, sufficient oil may leak to the surrounding environment so as to present a substantial health and/or safety hazard before detection occurs. Moreover, as more fuel escapes the location of the leak will be more difficult to determine.
Hydrocarbon leak detecting apparatus often have been constructed in a manner which requires their replacement or repair upon detection of a leak, that is, once contacted by a hydrocarbon fuel, the detection device, or its key components, must be replaced before the detector can be used again. Another problem is that in most storage tank farms, there will be considerable ground water present, and any detector must be capable of distinguishing between ground water and hydrocarbons and capable of functioning without being overwhelmed by ground water in order to avoid false detection signals. Finally, most hydrocarbon detectors are based upon sensing hydrocarbons in either a vapor state or a liquid state, but not both. The vapor-based sensors, therefore, tend to be overrun by ground water and liquids, and the liquid-base sensors tend to be insensitive to the presence of vapor.
In recent years, many attempts have been made to employ the unique and varied light transmission properties of optical fibers in detecting apparatus. In communication cable applications, the microbending of an optical fiber has been used to detect the location of moisture or ground water entering the cable. In U.S. Pat. No. 4,596,443 to Diemeer, et al., an unspecified swelling agent is positioned inside the cable and is mechanically coupled to press a ribbed-shaped pattern against the optical fiber upon swelling of the agent. Optical time domain reflectometry or a back-scatter technique is used to locate the position of the microbend along the fiber, and thus the position of water leaking into the cable. Such a system, however, is designed for sensing the presence of water, rather than to be insensitive to water and to detect the presence of hydrocarbon fuels. Reversibility of the swelling agent's expansion also is not suggested or disclosed in the Diemeer, et al. patent.
U.S. Pat. No. 4,590,462 to Moorehead also employs microbending of an optical fiber in a detection unit, and the Moorehead device is used to detect hydrocarbon fuels. A rotary actuator is mechanically coupled to an optical fiber to produce microbending of the fiber. The rotary actuator includes a spring mechanism having stored energy which is released upon degradation of shear pins under the action of hydrocarbons. Thus, when the hydrocarbon analyte is present in sufficient quantity to degrade the shear pins, the spring is released and the optical fiber displaced to produce a microbend that can be sensed by optical time domain reflectometry. This approach, however, clearly is not reversible since it depends upon destruction of the shear pins upon contact with the hydrocarbon.
A number of prior art fiber optic-based detector systems have been based upon the coupling of the evanescent wave traveling down the exterior of the fiber optic core. Thus, U.S. Pat. Nos. 4,270,049, 5,138,153, 5,144,690 and 5,168,156 are all based on evanescent wave phenomenon. In the patent to Tanaka et al., U.S. Pat. No. 4,270,049, a fiber optic sensor assembly is employed in which the light transmitted down the fiber optic core is reduced in intensity due to adhesion of an analyte, such as a hydrocarbon fuel, to the core. The core is clad with material having an index of refraction which is less than the core index of refraction, and contact and adhesion of the analyte to the cladding results in an increase in clad index of refraction which results in a reduction in the light transmitted along the core. The Tanaka et al. patent also teaches the use of silicon resins as a cladding which will be damaged or broken down by hydrocarbons.
In U.S. Pat. No. 5,138,153 to Gergely et al., a distributed fiber optic sensor based upon evanescent effects is disclosed in which the cadding has an index of refraction less than the core and the cladding is sensitized to the analyte. When the analyte contacts the cladding, it increases the index of refraction of the cladding above the core to thereby couple the light transmitted in the core to the evanescent wave. The Gergely et al. patent employs its sensor system in a hydrocarbon tank farm, but the cladding is selected to undergo an increase in the index of refraction. Optical time domain reflectometry is used to locate leaks, and both continuous and pulsed light can be employed to sense liquids and vapors having analytes which will react with the cladding. The patent to Gergely et al., however, has no disclosure as to cladding materials which are suitable for use on the fiber optic strand.
U.S. Pat. No. 5,144,690 to Domash discloses a fiber optic sensor system in which coating patterns are provided which induces strain on a dual core optical fiber. Evanescent wave coupling between cores is sensed. This system is not disclosed as being intended for hydrocarbon fuel detection.
U.S. Pat. No. 5,168,156 to Fischer et al. employs a fiber optic sensor assembly in which three fibers are used with one acting as an input, and the other two acting as a reference fiber and a signal fiber. The three fibers are coupled together optically and the sensor fiber is stripped with cladding and exposed to an analyte to be sensed. Light attenuation as a result of the analyte affect on the evanescent wave on the unclad fiber is detected as compared to the reference fiber, which is clad and shielded from the analyte.
Fiber optic detectors also have been based upon the interpositioning of a sensor material along the length of a fiber optic core so that transmission and/or reflection measurements will indicate when an analyte is present at the material interposed along the core. U.S. Pat. Nos. 4,842,783, 5,015,843 and 5,164,588 are examples of this approach. In U.S. Pat. No. 4,842,783 to Blaylock, a fiber optic sensor assembly is provided in which a polymeric gel is cross-linked in situ onto the end of the fiber and a dye absorbed into the gel, which preferably is swellable to absorb the dye. The dye in the gel is selected to be responsive to an analyte to be sensed. The dye, for example, can be fluorescent but the system is not designed for use with hydrocarbons. Dyes, however, tend to be suitable only in irreversible chemical reaction which require replacement of the sensor once the analyte is exposed to it.
U.S. Pat. No. 5,015,843 to Seitz et al. is directed to a fiber optic system in which polymer swelling is used to mechanically or physically displace a reflective surface coupled to the fiber optic core and thereby influence light transmission back to the detector. The system requires a relatively high concentration of analyte to be effective, and in order to enhance a sensitivity and minimize this disadvantage, the system preferably is miniaturized. Numerous polymers are discussed for sensing various products, none is disclosed as being reversible in its reaction with hydrocarbon fuels.
In U.S. Pat. No. 5,164,588 to Marcus, a distributed sensor system is provided in which reflector/transmission couplings and analyte sensors are interposed in series and alternating along an optical fiber strand. Light pulses pass through the sensors, and in part through the connectors and in part back to a detector, and the pulses can be used to detect environmental or analyte effects on the sensors. Many analytes may be sensed but hydrocarbon fuel sensing is not disclosed.
Notwithstanding the success of the various optical fiberbased detectors in detecting a wide range of analytes, detecting the presence of liquid and/or vapor hydrocarbon fuels, and distinguishing the same from ground water, by a detector system which is reversible and reusable through many cycles without significant hysteresis loss remains a substantial problem.
An alternative to optical fiber detection has also been employed which is currently being marketed under the trademark GORE-TEX cable, for example, by W. L. Gore and Associates of Phoenix, Ariz. The GORE-TEX coaxial cable has a hydrocarbon fuel absorbent media (expanded PTFE) situated between a central conductor and an outer, perforated, cylindrical conductor or shield. Pulses of RF energy are transmitted down the coaxial cable and are reflected back at the location of absorbed hydrocarbon. Time domain reflectometry is used to locate the position along the cable at which hydrocarbons are absorbed.
The GORE-TEX cable, however, cannot distinguish between Jet-A, gasoline or diesel fuel. It will not detect hydrocarbons in a vapor state, it cannot generate an analog output signal, and it has location resolution and distance range limitations.
Accordingly, it is an object of the present invention to provide an apparatus and method for detecting the presence of hydrocarbon fuels in either a liquid or a vapor state which can discriminate between such fuels and water and is suitable for use for multiple detection cycles.
A further object of the present invention is to provide a hydrocarbon fuel detection apparatus and method which is easy to install, requires minimum maintenance, is inexpensive to construct, is easily adjusted and can differentiate between different hydrocarbon fuels and can distinguish such fuels from ground water.
The hydrocarbon fuel detection apparatus and method of the present invention has other objects and features of advantage which will become apparent from, and are set forth in more detail in, the accompanying drawing and the following description of the Best Mode of Carrying Out the Invention.